STEP 3. Apply lime, gypsum and phosphorus (if necessary), and rip and cultivate the soil (continued) Why do some soils need lime?
The feeder roots in the surface soil need soft, stable, well-drained soil, with a pHwater of between 5.8 and 6.5.
Soil with a pHwater less than 5.8 is acid. In acid soils excess aluminium and possibly manganese become available and are toxic to roots. The roots become stunted and unable to take up sufficient water and nutrients.
Other nutrients such as calcium and magnesium may be present in acid soil but become unavailable to roots.
Also, phosphorus and sulphur may be present in acid soil, but they combine with aluminium to form aluminium phosphate and aluminium sulphate compounds which are not available to roots.
Why do some soils need gypsum?
Gypsum, which is calcium sulphate, is sometimes needed to soften surface soils and to improve their structure.
With gypsum, the soluble calcium swaps with some of the exchangeable cations, such as sodium and magnesium. Gypsum does this better than lime because gypsum is more soluble than lime. Gypsum does not affect soil pH, but lime does.
Cations (positive ions) such as sodium and calcium, exist in soil as either exchangeable cations (loosely bound to clay particles), or soluble cations (dissolved in soil water). The soluble cations often swap with exchangeable cations in soil.
When exchangeable sodium makes up more than 5% of the total exchangeable cations, and there are low concentrations of soluble cations, the soil is sodic and unstable.
When sodic soils are wetted, the clay particles push each other apart. At first the aggregates swell and decrease the size of the pores. On further swelling, small groups of clay particles separate from the larger aggregates and become suspended in the soil water until the clay particles block the small pores. This is called soil dispersion.
STEP 4. Hill up the surface soil (continued next issue)
See this article in Tree Fruit March 2015
